This invention relates to nondestructive testing of metals and in particular to the thickness measurement of a magnetic flux conductive base material that has been applied over a magnetic flux conductive base material of a permeability different from the permeability of the clad material.
Vessels, piping and other parts used in the nuclear, petrochemical, and other industries that require corrosion or heat protection are often fabricated of a clad material, chosen for its particular properties, over a base material. The clad construction is more economical than fabricating the entire vessel, pipe or other part entirely from the clad material. The clad material is often applied to the base material of the vessel, pipe or other part by a welding process after which the clad material is machined or ground to form the final surface. Although the original clad thickness can be estimated from the particular welding process used to apply the clad material, it is difficult to estimate the thickness of clad material remaining after machining or grinding.
Ultrasonic methods have been used to measure the thickness of clad material by detecting the interface between the clad material and the base material. Ultrasonic methods detect the interface better when the clad material is not applied well to the base material than when the clad material is applied well to the base material. The more desirable condition where the clad material is applied well to the base material does not result in reliable ultrasonic determination of the thickness of the clad material.
A method of determining clad thickness by pulling a magnet resting on the surface normal to the surface of a clad material that has been applied over a base material has also been used. This method measures the break-away force and correlates that force to clad thickness. This method of determining clad thickness is limited to applications where the permeability of the clad material is low and the permeability of the base material is high.
Therefore, a need exists for a nondestructive method and apparatus for determining the thickness of a magnetic flux conductive clad material that has been applied over a magnetic flux conductive base material of a permeability different from the permeability of the clad material.